The desirability of replacing conventional engineering materials has led material scientists and engineers to look at new materials or the processing of existing materials to enhance their mechanical properties. It is the latter route which is the subject of this invention.
The carbon-carbon bond is the strongest bond known to man. If one could somehow process materials containing this bond so that all the carbon-carbon bonds were aligned in the same direction and closely spaced relative to one another, this would yield a material with very high modulus and strength. The important question is which materials possess the possibility of being processed in such a way as to take advantage of the inherent strength of the carbon-carbon bond.
Scientists have known for a long time that the polyethylene molecule, because of its simple structure, and high density of carbon-carbon bonds per unit area, is a material with one of the highest theoretical values of tensile modulus and strength. The theoretical modulus and strength are thought to be approximately 300 GPa and 20 GPa, respectively. This is quite remarkable if one considers that the modulus and strength of steel are 200 GPa and 2 GPa, respectively. The challenge has been to process polyethylene in such a way as to obtain a material which comes as near as possible to the theoretical values of modulus and strength.
Conceptually, achieving a high modulus and strength involves uncoiling the molecule and stretching it out to align the carbon-carbon bonds in the backbone. In practice, however, this is very difficult due to the presence of inter- and intramolecular entanglements which hinder the extent to which the molecules can be drawn out.
There are essentially three routes for the expression of oriented polyethylene articles. These are solid, solution, and melt phase processing. In each process, the polyethylene molecules are in various morphological states prior to orientation.
In solid phase processing, orientation takes place while the material is below the melting point or in the crystalline state. Because of this, operating pressures are very high, and consequently the process is relatively slow. In addition, the process is usually discontinuous. Therefore, attempts at commercializing this orientation route have been very limited. The process does, however, produce articles of relatively high values of modulus and strength; typical values are 80 GPa and 1 GPa, respectively. The one advantage of solid phase processing is the ability to produce complex shapes because of the long relaxation times of polyethylene in the solid phase.
Prior to the advent of low entanglement density reactor grade polyethylenes, one of the limitations of solid phase processing was that the entanglements present during the manufacture of polyethylene were still present during processing. To take advantage of the low entanglement density, however, the polymer must be processed in the solid phase since any excursion above the melting point destroys the low entanglement morphology.
In solution processing, the degree of entanglement is reduced by dissolving the polyethylene molecules in a suitable solvent at such a concentration that the individual polyethylene molecules are just barely in contact with one another. The next step is to process this solution, remove the solvent and draw out the resulting precursor to very high draw ratios, this being possible due to the reduction in the entanglement density. This method is essentially the basis of all the so-called "gel-spinning" processes. The technique produces fibers of very high modulus and strength, typical values being 100 GPa and 3 GPa. This process is, however, very expensive because of the large amount of solvent recovery required. Furthermore, the method is restricted to the production of articles with at least one small dimension such as fibers or tapes, due to mass transfer limitations.
The remaining route for the production of oriented articles from polyethylene is via the melt phase. This technique is not as well developed as the solid or solution phase methods. The principle reason for this is that, because of the low thermal conductivity of polymers combined with the short relaxation times of conventional extrusion-grade polyethylene molecules in the melt, flow-induced orientation cannot be locked into the final structure to produce articles with significant values of modulus and strength. The advantage of a melt phase process is, however, the possibility of using a conventional extruder which might allow one to produce continuous products at commercial rates of production.
Recent work has shown that certain grades of high molecular weight polyethylene have the ability to be melt-drawn to produce articles of relatively high modulus and strength; typical values are 80 GPa and 1 GPa, respectively. The source of these high mechanical properties is the unique morphology produced when these grades of polyethylene are oriented in the melt phase at critical conditions of temperature and strain rate.